CODEWITHSUNDEEP
c#cpointerspinvokemarshalling
elleciel
elleciel

Reputation: 2557

Marshalling output pointers during P/Invoke calls from C# to C

I have a C library with this function that returns 4 output parameters:

void __declspec(dllexport) demofun(double a[], double b[], double* output1, double* output2, double res[], double* output3)

And I wrote a C# wrapper to call the function:

namespace cwrapper
{
    public sealed class CWRAPPER
    {    
        private CWRAPPER() {}

        public static void demofun(double[] a, double[] b, double output1,
                            double output2, double[] res, double output3)
        {   // INPUTS: double[] a, double[] b
            // OUTPUTS: double[] res, double output1, double output2
            // Arrays a, b and res have the same length
            // Debug.Assert(a.length == b.length)

            int length = a.Length;
            CWRAPPERNative.demofun(a, b, length, ref output1, ref output2,
                         res, ref output3);
        }
    }

    [SuppressUnmanagedCodeSecurity]
    internal sealed class CWRAPPERNative
    {
        private CWRAPPERNative() {}

        [DllImport("my_cwrapper.dll", CallingConvention=CallingConvention.Cdecl,
             ExactSpelling=true, SetLastError=false)]
        internal static extern void demofun([In] double[] a, [In] double[] b,
                             int length, ref double output1, ref double output2,
                             [Out] double[] res, ref double output3);
    }
}

Everything works fine when I call the CWRAPPERNative.demofun method. However, when I call the CWRAPPER.demofun method, only the double[] res is passed through correctly. The output parameters output1, output2 and output3 are unchanged after the call.

// ...
// Initializing arrays A and B above here

double[] res = new double[A.Length];
double output1 = 0, output2 = 0, output3 = 0;

// Works partially: output1 to 3 unchanged
CWRAPPER.demofun(A, B, output1, output2, res, output3);

// Works correctly: all outputs are changed
CWRAPPERNative.demofun(A, B, A.Length, ref output1, ref output2, res, ref output3);

I'm guessing that I'm marshalling the pointer arguments wrongly, but I cannot figure out the fix. Anyone knows a solution? Thanks!

Upvotes: 1

Views: 4294

Answers (2)

groverboy
groverboy

Reputation: 1135

All double parameters to CWRAPPER.demofun are non-volatile, i.e. the values cannot be changed by the function. To fix this you need a change like this:

public static void demofun(double[] a, double[] b,
  out double output1, out double output2, double[] res, out double output3)

There's another problem though. The C function's 5th parameter double res[] is an input parameter but your P/Invoke declaration has [Out] double[] res (an output parameter). I'd rewrite the P/Invoke declaration like this:

[DllImport("my_cwrapper.dll", CallingConvention=CallingConvention.Cdecl,
  ExactSpelling=true, SetLastError=false)]
static extern void demofun([In] double[] a, [In] double[] b, [In] int length,
  [Out] double output1, [Out] double output2, [In] double[] res, [Out] double output3);

Example method implementation with out parameter; OutOutTester shows that a method with out parameters need not assign their values directly:

public static void OutTester(out int a)
{
    a = 1;
}

public static void OutOutTester(out int a, out int b)
{
    OutTester(out a);
    b = 1;
}

Upvotes: 0

Nican
Nican

Reputation: 7935

You are forgetting to pass the values by reference inside of demofun:

public static void demofun(double[] a, double[] b, ref double output1,
   ref double output2, double[] res, ref double output3)

The values is changing inside of the method, but not being modified to the original caller.

Upvotes: 1

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